[AWDFTW]

Miles on vehicle?
Has the ATF even been changed? If so, with what fluid?

 

[AWDFTW]

You might call and ask.

For vehicles that came with the Subaru ATF HP fluid, mixing in the older style Dex III fluid can cause some shifting issues.

But the 4EAT and 5EAT both seem to be gear hunters.

 

[AWDFTW]

The owner's manual states that the H6 is designed for optimum performance on premium fuel, although it can compensate for lower octane. You might try a few tanks of premium and see if it adjusts.

Unlike the H4 which runs fine on regular and doesn't have anything to gain by higher octane fuel if pinging/knocking is not present.